I have a relatively pedestrian knowledge of electronics, but was interested to know if I needed (as recommended in several articles I've seen from time to time) to put a 680 ohm resistor across the output terminals of my LA-3a, which has a low (600 ohms) input and output impedance, to properly match to my 192 I/O when using it as an I/O plug-in. I cannot find any specs which indicate the 192's input and output impedance anywhere. Supposedly, 'ringing' will occur if there is a mismatch, and I don't know if it's something that would be blatantly obvious or even exactly what 'ringing' sounds like. I'm imagining it's a resonance or edge-of-feedback kind of sound. Am I close? I don't hear anything like that, yet. Is it gain related or omnipresent if there is indeed a Z mismatch? And related to this issue, where should the impedance switch on the back of my UA 6176 be set to? Thanks for listening...

Old does not mean bad sound quality. There is a reason why people still use LA-3a's and all the plug in comapanies try to emulate those classic pieces of gear. I would pick a real LA-3a over a plug in anyday. I wish people didn't buy into the mentality that digital is always better and that new is always better than old.
JJ

There is a transformer in the LA-3A's output circuit. It will have flattest frequency response and least "ringing" when it is terminated by the proper impedence. Connecting a 600 ohm resistor (680 is close enough) will ensure best technical performance, in terms of response, sometimes distortion, and correct output level. Older audio gear provided 600 ohm input impedence, but modern gear does not, thus the resistor is used to compensate.

The ringing is not like feedback, is almost always above 20Khz, and therefore it's effects are usually very subtle to the ear. The frequency response issues are usually more audible, although I do not know specifically how the LA-3A reacts. Many URIE devices were well designed and sounded very good unterminated. Often, the high frequencies are emphasized when not terminated but otherwise good.

I agree you should put a 600 (620-680) ohm resistor across the output or you may experianve low freq. roll off or Hi end abnormalities.. I seen it happen with this type of euipment..
Some times you may even have to match input inpedence with a 'U" pad.. Hi enough in for the equipment driving it but 600 ohms to the input of the LA3 etc.. Back in the days most equipment was designed with 600 ohms in and out.

I have been told that LA-3A's really all need to have a very basic mod to keep the noise down. I have used them for years with great luck, and then used a pair in one particular studio that were very noisy and hissy. A guru tech at a world renowned studio told me that most likely it just needed the mod, which he had done to about 12 of that studio's units. I'm not too technical, but I think it was just a resistor.

Also keep in mind the unbalanced outputs of the 192 when hooking outboard gear directly to the 192's outs. This can cause problems.